Positive displacement superchargers of the Roots rotor type are sometimes used in automotive engines to increase the cylinder air charge and thus provide for increased engine output. The rotors may be formed with helical lobes to provide for axial air flow from an end wall inlet to an upper outlet adjacent an opposite end wall in order to improve efficiency and reduce noise.
There is commercially available a family of twisted, or helical, rotor Roots superchargers for use by engine manufacturers. These are based on a nominal 103 mm rotor diameter. Various displacements are produced by varying the lengths of the rotors. However, a different helix angle is used for each length, as it had been believed that a 60 degree offset between the front and rear faces of the rotors was optimum, independent of the rotor length.
It had also been believed that 191 mm was as long as rotors could be made for the 103 mm family due to thermal considerations between the inlet and outlet and to deflections of the rotor components.
The 191 mm rotor set results in a displacement of 112 cubic inch/revolution. For this design, the face offset from one end of the rotor to the other equals the previously considered optimum angle of 60 degrees, resulting in a helix angle twist of essentially 0.314 deg/mm. In addition, the seal time, expressed as the angular distance from closing of the inlet port connection from one rotor cavity to the opening of that cavity to the exhaust port, was 67 deg.
An engine application for a new project required greater air flow than the 112 cubic inch rotors could provide, so the design of a longer rotor was explored. Based upon earlier experience, it was believed that the 60 deg front-to-rear face offset would have to be maintained. However, this would have required new and expensive extrusion dies and rotor hobbing tools.
The inventor proposed instead that the rotors be simply lengthened to 208 mm without changing the helix angle of 0.314 deg/mm in order to minimize the expense. This yielded a 65.3 deg front-to-rear face offset and would yield a nominal 8.9% increase in displacement over the 112 cubic inch unit. This configuration resulted in a displacement of 122 cubic inches per revolution.
This new 122 cubic inch displacement unit did not have the 60 degree rotor face offset long believed to be optimal, but the unit was prototyped and tested as it was the most cost effective method to obtain the desired increased rotor displacement.